1. Field of the Invention
The present invention generally relates to mobile communication systems and mobile station apparatuses, and, more particularly, to a mobile communication system in which a radio zone where a mobile station is to enter a wait state is determined by the electric field intensity of a received radio wave that arrives from a radio base station, and to a mobile station apparatus that receives communication service by accessing such a mobile communication system.
The present invention also relates to a mobile communication system wherein a mobile station selects a base station to which a request for a message channel is to be issued, based on announcement information from the base station.
2. Description of the Related Art
Recently, a variety of mobile stations including an automobile mobile station and a portable mobile station access a mobile communication system. With a competition between a plurality of carriers as a background, the number of mobile stations is increasing.
A mobile communication system may have a large-zone construction in which a base station covers a relatively wide service area or a small-zone construction in which a plurality of base stations cover a service area.
In a mobile communication system, radio base stations operated on a small transmission power and forming microcells or picocells are provided at the center of a big city characterized by a particularly large volume of traffic so that a multilayer cell is formed.
Radio base stations operated on a small transmission power are also provided in an underground passage and a tunnel in order to enlarge a radio zone sufficiently to eliminate a dead zone.
FIG. 1 shows a construction of a conventional mobile communication system.
Referring to FIG. 1, radio base stations 611 and 612 form adjacent radio zones 621 and 622, respectively. In the radio zone 621, a radio base station 613 forming a microcell 63 and a radio base station 614 forming a picocell 64 are provided so as to form a multilayer. Mobile stations 651-65N are located movable in the radio zones 621 and 622, the microcell 63 and the picocell 64.
In the radio base station 611, an antenna 661 is connected to an antenna terminal of a transmission and reception unit 681 via a common antenna equipment unit 671. A control terminal of the transmission and reception unit 681 is connected to an input and output terminal of a base station control device 691. A line terminal provided in the transmission and reception unit 681 and a communication port of the base station control device 691 are connected to a control station (not shown) via a transmission device 701 and a communication link 711.
Since the radio base stations 612-614 have the same construction as the radio base station 611, the corresponding elements are designated by using suffixes 2-4 in the description below, and the illustration and description thereof are omitted.
An antenna 721 of the mobile station 651, is connected to an antenna terminal of a transmission and reception unit 741 via a common antenna equipment unit 731. A modulated input and a demodulated output of the transmission and reception unit 741 are connected to a microphone 751 and a speaker 761, respectively. A control terminal of the transmission and reception unit 741 is connected to a control terminal of a control unit 771. An input and output terminal of the control unit 771 is connected to a display and operation unit 781.
Since the construction of the mobile stations 652-651 is the same as the mobile station 651, the corresponding elements are designated by using suffixes 2-N in the description below, and the illustration and description thereof are omitted.
The base station control device 691 of the radio base station 611 in the above-described mobile communication system is directed by a center (not shown) via the communication link 711 and the transmission device 701 to generate announcement information including a wait enabled level and a wait disabled level as shown in FIG. 2 and transmits the announcement information to a predetermined control radio channel (hereinafter, simply referred to as a control channel) via the transmission and reception unit 681, the antenna common equipment unit 671 and the antenna 661.
In addition to the wait enabled level and the wait disabled level, the announcement information also includes a message type identifying transmitted information as the announcement information, a mobile station transmission power specification specifying a transmission power of the mobile station, a location code indicating a location of the radio zone (service area), and the like. However, the information items other than the wait enabled level and the wait disabled level does not have to do with the present invention, and the description thereof is omitted.
The control unit 771 of the mobile station 651 maintains a control channel table listing all the radio channels over which the announcement information described above is transmitted from the radio base stations forming the respective radio zones to which the mobile station 651 may be located. Further, upon power-on, the control unit 771 measures sequentially the electric field intensity L1 of the control channel registered in the control table channel by controlling the transmission and reception unit 741 ((1) of FIG. 3). A determination is then made as to whether or not the measured electric field intensity is greater than a predetermined threshold level Lth ((2) of FIG. 3).
The control unit 771 registers the control channels for which a determination that the measured electric field intensity is greater than the threshold level Lth is obtained, in a prescribed area (hereinafter, referred to as a candidate zone register) in a main memory. The electric field intensity of the registered control channel is also registered in the candidate zone register so that the correspondence between the control channel and the associated electric field intensity is specified ((3) of FIG. 3).
When the control unit 771 has completed a series of the above-described processes (hereinafter, referred to as a measurement process) for all the control channels registered in the control channel table, the control unit 771 determines whether or not the candidate zone register stores any control channel ((4) of FIG. 3). If an affirmative answer is given, the control unit 771 sorts the entries in the candidate zone register according to the ascending order of the electric field intensity ((5) of FIG. 3).
The control unit 771 refers to the candidate zone register when the sorting process has been completed and receives the announcement information via the individual control channels registered in the candidate zone register ((6) of FIG. 3). The control unit 771 measures the electric field intensity again ((7) of FIG. 3). Further, the control unit 771 compares the electric field intensity L2 and the wait enabled level Lth included in the announcement information ((8) of FIG. 3). When the former is lower in level than the latter, a similar comparison is conducted for the other control channels registered in the candidate zone register ((9) of FIG. 3). Hereinafter, a series of processes performed subsequent to the measurement process is referred to as a zone determination process. The control unit 771 restarts the measurement process when the electric field intensity is found to be below the wait enabled level Lth for all the control channels subjected to the comparison ((10) of FIG. 3).
If the electric intensity field of any of the control channels is found to be equal to or exceeds the wait enabled level, the control unit 771 establishes that control channel as a control channel for the radio zone in which the mobile station is to register its location, issues a call and receives an incoming call ((11) of FIG. 3). Thereafter, the control unit 771 enter a wait state ((12) of FIG. 3).
The operations relating to registering of a location, issuing of a call or receiving of an incoming call performed by the mobile station 651 and the radio base station 611 are not directly related to the present invention, and the description thereof will be omitted below.
The operation performed by the radio base stations 612-614 is the same as the above-described operation of the radio base station 611, and the description thereof is omitted. Also, the operation performed by the mobile stations 652-65N is the same as the above-described operation of the mobile station 651, and the description thereof is omitted.
In the conventional system described above, if the mobile station 651 is located inside the microcell 63 and near the border with respect to the radio zone 621, as indicated by the broken line in FIG. 4, and if the electric field intensity of the radio wave arriving from the radio base station 611 is higher than that of the radio wave received from the radio base station 613, the mobile station 651 is set up for a wait in the radio zone 621. As illustrated, the radio zone 621 is formed outside the microcell 63 in which the mobile station 651 is actually located.
In such a case, the microcell 63 and the picocell 64 provided with the aim of handling a large volume of traffic are not accessed by the mobile station 651. The traffic otherwise handled by the microcell 63 is imposed as a load on the radio base station 611, thus causing the operating efficiency and the service quality to be dropped.
When the propagation loss of the radio wave that arrives at the mobile station 651 from the radio base station 611 increases due to the propagation characteristic of the radio transmission channel varying depending on the speed of the mobile station mobile station 651 and the path of the movement, the mobile station 651 may leave a zone when it is not necessary and performs the measurement process. Even when a call originated in the mobile station 651 becomes a successful call so that a conversation is established without the mobile station 651 leaving the zone, the speech quality may be dropped if there is a large propagation loss, with the result that switching of message channels occur too frequently during the conversation.
Given that the microcell 63 or the picocell 64 is provided in order to serve the dead area located inside the radio zone 621, the mobile station 651 may be set up for a wait in a radio zone located outside the cell (the microcell 63 or the picocell 64) in which the mobile station 651 is actually located. In this case, the microcell 63 or the picocell 64 remain unused so that the dead area is not efficiently served, a first aspect of the problem with the conventional mobile communication system.
A description will now be given, with reference to FIG. 5, of the flow of call origination operation in the conventional mobile communication system, in order to explain a second aspect of the problem with the conventional mobile communication system. In the following explanation of the second aspect, the PDC mobile communication system employed primarily in Japan is assumed.
In the conventional mobile communication system, when the user of a mobile station turns the power on (ST701), the mobile station measures a reception level of a perch channel provided for a base station (ST702). A perch channel is a term used in the PDC mobile communication system to refer to a channel provided for each of the base stations to allow a mobile station to determine a reception level with respect to the base station. In the PDC system, a perch channel is mainly used to transmit announcement information from the base station to the mobile station. The claims refer to a perch channel as a reception-level determining channel. If it is determined that the reception level exceeds a predetermined level (YES in ST703), the mobile station sorts the perch channels according to the ascending order of reception levels and stores the perch channels and the associated reception levels in a memory provided in the mobile station (ST704). A determination is then made as to whether the reception levels in all the perch channels have been measured (ST705).
If no perch channels and associated reception levels are stored in the memory (NO in S706), the mobile station displays an out-of-the-zone message in a display device (S707). When perch channels and associated reception levels are stored in the memory (YES in S706), the mobile station measures the reception level in the perch channel having the highest reception level according to the memory. If it is determined that the measured reception level exceeds a wait enabled level contained in the announcement information from the base station, the mobile stations is set up for a wait in the base station providing the highest reception level (S708).
The mobile station in a wait state sends a call request to the base station in which it is set up for a wait (S709). The mobile station notifies the base station of the perch channel codes and the reception levels provided by the base stations other than the notified base station.
If the level of reception from the mobile station issuing the request exceeds a predetermined level that enables assigning of a message channel (YES in S710), and if there is an unused message channel (YES in S711), the base station receiving the request from the mobile station assigns a message channel to the requesting mobile station (S712). If the level of reception from the mobile station is below the predetermined level (NO in S710), or if there is no unused message channel (NO in S711), the base station receiving the request refers to the reception levels of the adjacent base stations reported by the mobile station and designates one of the adjacent base stations as a base station for the requesting mobile station (S713).
The mobile station that receives, from the base station, a signal for assigning a message channel can use the assigned message channel to establish a call connection with the corresponding base station (S714).
Thus, the mobile station located at a point where there is an overlap of a plurality of zones formed by respective base stations is set up for a wait in the base station whose perch channel provides the highest reception level. When originating or receiving a call, the mobile station issues a request for a message channel to the base station in which the mobile station is set up for a wait.
However, it is not always best for the mobile station in the mobile communication system to issue a request for a message channel to the base station providing the highest reception level in the perch channel.
A relatively large service area formed, for example, by the base station A as shown in FIG. 31 is assumed. This large-zone construction formed by the base station A may include an area characterized by a concentrated traffic, resulting in a shortage of message channels in that area. In order to remedy this shortage, the base station B having a smaller transmission power and capable of forming a smaller-zone construction than the base station A is provided within the service area formed by the base station A. Provision of the base station B would serve the intended purpose if a mobile station within the zone formed by the base station B requests a message channel from the base station B.
However, a problem with the conventional mobile communication system is that the mobile station requests a message channel from the base station A providing a higher reception level than the base station B instead of requesting it from the base station B provided to handle an increased local traffic, with the result that the base station B remains unused to handle call originating or call incoming in the mobile station.